I keep getting these grey SMS Message pop-ups

The screen went black wth a grey box over it that said "SMS message" at the top and was entirely symbols. I clicked the option to see what it was but all it told me was it thought it was an attempt to hack my phone and that the sender would receive a recipe notification. It happened when my phone was open and when it was locked. It said they won't save. So, what are these? And is it actually from one of my contacts but screwed up? Or is it a phishing attempt? Because my friends have been asking me why I don't reply but I won't have messages from them